Synergistic Effect of Y Doping and Reduction of TiO2 on the Improvement of Photocatalytic Performance

Pure TiO2 and 3% Y-doped TiO2 (3% Y-TiO2) were prepared by a one-step hydrothermal method. Reduced TiO2 (TiO2-H2) and 3% Y-TiO2 (3% Y-TiO2-H2) were obtained through the thermal conversion treatment of Ar-H2 atmosphere at 500 °C for 3 h. By systematically comparing the crystalline phase, structure, morphological features, and photocatalytic properties of 3% Y-TiO2-H2 with pure TiO2, 3% Y-TiO2, and TiO2-H2, the synergistic effect of Y doping and reduction of TiO2 was obtained. All samples show the single anatase phase, and no diffraction peak shift is observed. Compared with single-doped TiO2 and single-reduced TiO2, 3% Y-TiO2-H2 exhibits the best photocatalytic performance for the degradation of RhB, which can be totally degraded in 20 min. The improvement of photocatalytic performance was attributed to the synergistic effect of Y doping and reduction treatment. Y doping broadened the range of light absorption and reduced the charge recombination rates, and the reduction treatment caused TiO2 to be enveloped by disordered shells. The remarkable feature of reduced TiO2 by H2 is its disordered shell filled with a limited amount of oxygen vacancies (OVs) or Ti3+, which significantly reduces the Eg of TiO2 and remarkably increases the absorption of visible light. The synergistic effect of Y doping, Ti3+ species, and OVs play an important role in the improvement of photocatalytic performances. The discovery of this work provides a new perspective for the improvement of other photocatalysts by combining doping and reduction to modify traditional photocatalytic materials and further improve their performance.


Introduction
Among the various semiconductors, titanium dioxide (TiO 2 ) is considered a useful photocatalyst for the treatment of water pollution and water splitting, owing to its minimal toxicity, strong oxidation capacity, and ready availability [1][2][3][4][5][6]. In recent decades, many research articles have reported the performance, synthesis methods, as well as the reaction mechanisms of TiO 2 in photocatalytic systems [7][8][9]. The crystalline phase of TiO 2 remarkably affects its photocatalytic activity. Rutile, anatase, and brookite are the three main crystalline phases of TiO 2 [10]. The photocatalytic performance of pure anatase TiO 2 is better than that of rutile, whereas the brookite phase is unstable and does not show appreciable photoactivity [11]. However, the wide bandgap (E g ; anatase,~3.20 eV) limits light absorption to the UV region of the solar spectrum (~4% of the total solar irradiance), thus affecting photocatalytic activity [12]. Moreover, the high recombination of the photogenerated electron-hole (e − -h + ) pair has been proven to reduce photocatalytic efficiency [13]. Many approaches, including metal (Al 3+ , Zn 2+ , Fe 3+ , Ce 3+ , Mn 2+ , Ag + ) [14][15][16] and nonmetal doping [17], the deposition of noble metal (Au, Pt) [18,19], and construction of heterojunctions with other semiconductors [20,21], have been explored to overcome these limitations.
Among these methods, rare earth (RE) metal doping is a popular technique to reduce the recombination rate of photogenerated carriers and shift the absorption wavelength to the visible region and increase the photoactivity of TiO 2 [22,23]. Furthermore, RE metal doping shows other benefits, such as the ability to concentrate pollutants at the TiO 2 photocatalyst surface, inhibit the phase transformation from anatase to rutile, and decrease the crystallite size [24,25]. Many studies reported that the concentration of RE metal dopant affects the photoactivity, with the optimum achieved at concentrations below 5 wt.% [25]. Among all the RE ions, yttrium (Y) ion is considered a typical dopant used to modify the electronic structure and optical properties of TiO 2 [26]. Specifically, it has been found that the Y-doped TiO 2 can reduce the recombination rate of photogenerated electrons/holes pairs, which improves the photocatalytic efficiency of TiO 2 [27]. The effect of Y-doped TiO 2 on photocatalytic activity has been recently reported in several studies [28,29].
In addition to RE metal doping, reduced TiO 2 , which is obtained by high-temperature treatments in various reducing atmospheres (e.g., vacuum, Ar, Ar-H 2 , and pure H 2 ) [30][31][32] or thermite reduction [33], reduction with sodium borohydride [34], and Ti 3+ self-doping [35,36], has shown tremendous potential as a photocatalyst in wastewater treatment and water splitting recently. The enhanced photocatalytic performance of reduced TiO 2 is related to a significant decrease in E g and increased light absorption due to the introduction of oxygen vacancies (O V ) or the formation of Ti 3+ centers in the TiO 2 lattice [34]. Current studies on reduced TiO 2 focused on the relationship between the Ti 3+ or O V concentration and photocatalytic activity of TiO 2 and comparison of the reduction degree caused by different reduction methods. However, the synergistic effect of RE doping and reduction treatment for TiO 2 photocatalysts has not been reported yet.
In this work, pure anatase TiO 2 , Y-doped TiO 2 , and their respective reduced products were prepared to verify the synergistic effect of Y doping and reduction treatment for TiO 2 photocatalysts. Pure anatase TiO 2 and Y-doped TiO 2 were obtained by a one-step hydrothermal method, reduced TiO 2 and reduced Y-doped TiO 2 were formed by heat treatment in an Ar-H 2 atmosphere at 500 • C for 3 h. The crystalline phase, morphological features, and photocatalytic properties of four types of TiO 2 were compared systematically. By comparing TiO 2 and Y-doped TiO 2 , the effect of RE doping on the improvement of photocatalytic performance could be obtained. Similarly, by comparing TiO 2 with reduced TiO 2 , the effect of reduction treatment by H 2 on the improvement of photocatalytic performance can be obtained. By comparing TiO 2 , which are simultaneously doped and reduced, with pure TiO 2 , Y-doped TiO 2 , and reduced TiO 2 , it can be verified the existence of a coupling effect of RE doping and reduction treatment on the improvement of photocatalytic performance. Ultimately, a positive effect of the combination of Y doping and reduction of TiO 2 on the improvement of photodegradation activity of organic pollutants under simulated sunlight irradiation was found, and their possible photocatalytic mechanism was proposed. The aim of this study was to develop an understanding of the synergistic effect of Y doping and reduction of TiO 2 on the improvement of photocatalytic performance.

Preparation of Pure TiO 2 and Y-Doped TiO 2 Nanoparticles
A total of 4.2 mL of TIP, 16.8 mL of absolute ethanol, and 1.68 mL of acetic acid were mixed under continuous stirring to form solution A. Solution B was composed of a certain amount of Y(NO 3 ) 3 ·6H 2 O dissolved in 21 mL of deionized water with a pH value adjusted to 2.5 by dilute HNO 3 (1 mol·L −1 ). The molar ratio of Y/Ti in solution B was set to 0, 1%, Nanomaterials 2023, 13, 2266 3 of 12 2%, 3%, 4%, and 5%, respectively. Then, solution B was added into solution A to obtain Y-doped TiO 2 sol. After stirring for 30 min, the Y-doped TiO 2 sol was transferred into a 70 mL Teflon vessel. The Teflon vessel was placed into stainless-steel autoclaves. Then, the autoclaves were put in an oven and heated to 180 • C for 12 h. Then, the autoclaves were cooled to room temperature after hydrothermal treatment. The final products were filtered, washed, and dried at 80 • C for 12 h.

Preparation of Reduced TiO 2 and Y-Doped TiO 2 Nanoparticles
TiO 2 and Y-doped TiO 2 nanoparticles were placed in a tube furnace, respectively, and then evacuated to a base pressure of about 0.5 Pa. The tube was filled with Ar-H 2 (95 vol %-5 vol %) atmosphere to normal pressure. The samples were heated at 500 • C with a heating rate of 5 • C /min for 3 h in Ar-H 2 flow to obtain the final reduced pure TiO 2 and Y-doped TiO 2 products by H 2 .

Photocatalytic Test
The photocatalytic activity test was conducted using RhB as a simulated pollutant and a 300 W Xe lamp as a simulated sunlight source (PLS-SXE300/300UV, Trusttech Co., Ltd., Beijing, China) to irradiate the pollutant. In a typical photocatalytic process, 10 mg of the catalyst was dispersed into 100 mL of the RhB solution (10 mg L −1 ) in a 200 mL double-layer reactor cooled by running water to maintain a temperature of 25 • C, and the mixture was magnetically stirred with 300 rpm in the dark for 1 h to achieve an adsorptiondesorption balance. Then, the xenon lamp was turned on, and 5 mL of the suspension was collected at every 10 min interval and centrifuged (9000 rpm, 5 min) to remove the catalyst powders. The total irradiation time of all samples was 1 h. The degradation rate of RhB was calculated by converting the absorbance into concentration using the dye standard curve by a UV-vis spectrophotometer.

Characterization
The phase structure of the products was determined using X-ray diffraction (XRD) (Bruker D8Advance diffractometer) with Cu K α radiation (λ = 1.5418 Å). The structural information of products was obtained using high-resolution transmission electron microscopy (HRTEM) by a JEM-2100F at 200 kV. X-ray photoelectron spectroscopy (XPS) measurements were carried out using the AXIS-Ultra DLD system. The UV-vis diffuse reflectance absorption spectra (DRS) of samples were used to determine the bandgap by the PE Lambda 950 spectrometer with BaSO 4 as reference. The Brunauer-Emmett-Teller (BET) method with a JW-BK200B was used to evaluate the specific surface area of the products.

Structure and Morphology Characterization
Pure TiO 2 and various Y-doped TiO 2 (1-5%) were prepared in advance to verify their phase composition and photocatalytic performance for the degradation of RhB. XRD was used to characterize and compare the crystalline phase of samples. All samples show the single anatase phase ( Figure S1a). The catalytic activities were evaluated by the degradation of RhB under simulated sunlight irradiation and the maximum photocatalytic performance for the degradation of RhB when the Y dopant concentration was 3% ( Figure S1b). Therefore, 3% Y-doped TiO 2 was selected as a typical research object for rare earth doping. Figure 1 shows the crystalline phase of pure TiO 2 , 3% Y-doped TiO 2 (3% Y-TiO 2 ), reduced TiO 2 by H 2 (TiO 2 -H 2 ), and reduced 3% Y-doped TiO 2 (3% Y-TiO 2 -H 2 ). All four samples have significant diffraction peaks representing the characteristic of a single anatase phase (PDF No. 211272). Anatase is the only crystalline phase present in the structure of TiO 2 -H 2 and 3% Y-TiO 2 -H 2 , indicating that no phase transformation occurs in the annealing process of H 2 reduction at 500 • C. After annealing, the colors of TiO 2 -H 2 and 3% Y-TiO 2 -H 2 change from white to gray black. The refraction of Y 2 O 3 is not observed in the XRD patterns of 3% Y-TiO 2 or 3% Y-TiO 2 -H 2 , indicating that the content of Y 2 O 3 is below the detection limit. No diffraction peak shift is observed for all Y-modified samples, demonstrating that Y 3+ species exists at the crystal boundary or surface rather than in the inner crystalline structure of TiO 2 . The increased diffraction peak intensities of TiO 2 -H 2 and 3% Y-TiO 2 -H 2 after annealing indicate the increased crystallinity of samples in comparison with those of pure TiO 2 and 3% Y-TiO 2 .
for rare earth doping. Figure 1 show the crystalline phase of pure TiO2, 3% Y-doped TiO2 (3% Y-TiO2), reduced TiO2 by H2 (TiO2-H2), and reduced 3% Y-doped TiO2 (3% Y-TiO2-H2). All four samples have significant diffraction peaks representing the characteristic of a single anatase phase (PDF No. 211272). Anatase is the only crystalline phase present in the structure of TiO2-H2 and 3% Y-TiO2-H2, indicating that no phase transformation occurs in the annealing process of H2 reduction at 500 °C. After annealing, the colors of TiO2-H2 and 3% Y-TiO2-H2 change from white to gray black. The refraction of Y2O3 is not observed in the XRD pa erns of 3% Y-TiO2 or 3% Y-TiO2-H2, indicating that the content of Y2O3 is below the detection limit. No diffraction peak shift is observed for all Y-modified samples, demonstrating that Y 3+ species exists at the crystal boundary or surface rather than in the inner crystalline structure of TiO2. The increased diffraction peak intensities of TiO2-H2 and 3% Y-TiO2-H2 after annealing indicate the increased crystallinity of samples in comparison with those of pure TiO2 and 3% Y-TiO2. The morphology and structure of all four samples are investigated by HRTEM. The low-magnification TEM images and statistical particle size distribution of TiO2, 3% Y-TiO2, TiO2-H2, and 3% Y-TiO2-H2 are shown in Figure 2a-d and their insert, respectively. All samples are primarily composed of dispersed circular, rectangular, and some irregularshaped nanoparticles with average sizes ranging from 8 nm to 11 nm. The grain sizes of TiO2-H2 and 3% Y-TiO2-H2 increase slightly but not significantly after annealing. Highmagnification TEM images show clear la ice fringes, which is indicative of the high crystallinity of TiO2 and 3% Y-TiO2 (Figure 2e,f). However, high-magnification TEM images of TiO2-H2 and 3% Y-TiO2-H2 illustrate a core-shell structure with a ∼1.5 nm-thick disordered surface shell (Figure 2g,h), which is not observed on the surfaces of TiO2 and 3% Y-TiO2 prepared by the single hydrothermal method. Four samples have la ice spacings of 0.34 nm in the same way as in Figure 2e, which is consistent with its (101) planes (Figure 2e-h). The elemental mapping images of 3% Y-TiO2-H2 with individual elements of Ti, O, and Y are shown in Figure 2i. Ti, O, and Y are uniformly distributed throughout the particle space, proving the presence of Y elements in Y-doped TiO2 samples. The surface areas of all products were estimated by the BET analysis; their N 2 adsorption-desorption isotherms are shown in Figure 3. The specific surface areas of pure TiO 2 and 3% Y-TiO 2 are 148.58 (Figure 3a) and 160.17 m 2 /g (Figure 3b), respectively, which can be related to changes in the morphology of the TiO 2 after doping with Y. Based on the XRD results, no diffraction peak shift is observed for 3% Y-doped TiO 2 , demonstrating that the Y 3+ species exists at the crystal boundary or surface rather than in the inner crystalline structure of TiO 2 . Therefore, it is speculated that the adhesion of Y 3+ species to the surface of particles increases the roughness of the particle surface, resulting in an increase in the specific surface area. The specific surface areas of TiO 2 -H 2 and 3% Y-TiO 2 -H 2 are 85.94 ( Figure 3c) and 117.42 m 2 /g (Figure 3d), respectively, which is lower compared to pure TiO 2 and 3% Y-TiO 2 . This may be attributed to the grain growth and slight aggregation caused by high-temperature annealing treatment, which is consistent with the results of the statistical particle size distribution in TEM images. The surface areas of all products were estimated by the BET analysis; their N2 adsorption-desorption isotherms are shown in Figure 3. The specific surface areas of pure TiO2 and 3% Y-TiO2 are 148.58 (Figure 3a) and 160.17 m 2 /g (Figure 3b), respectively, which can be related to changes in the morphology of the TiO2 after doping with Y. Based on the XRD results, no diffraction peak shift is observed for 3% Y-doped TiO2, demonstrating that the Y 3+ species exists at the crystal boundary or surface rather than in the inner crystalline structure of TiO2. Therefore, it is speculated that the adhesion of Y 3+ species to the surface of particles increases the roughness of the particle surface, resulting in an increase in the specific surface area. The specific surface areas of TiO2-H2 and 3% Y-TiO2-H2 are 85.94 (Figure 3c) and 117.42 m 2 /g (Figure 3d), respectively, which is lower compared to pure TiO2 and 3% Y-TiO2. This may be a ributed to the grain growth and slight aggregation caused by high-temperature annealing treatment, which is consistent with the results of the statistical particle size distribution in TEM images.  Based on the survey scanning the XPS spectra of four samples, all labeled peaks were attributed to Ti 2p and O 1s ( Figure S2). Furthermore, a weak Y 3d peak is observed at~158 eV, indicating the presence of Y 3+ species on the surface of 3% Y-TiO 2 and 3% Y-TiO 2 -H 2 samples. The high-resolution XPS spectra of Ti 2p for four samples are shown in Figure 4a-d. TiO 2 and 3% Y-TiO 2 exhibit two peaks at 458.4 and 464.2 eV, which are attributed to 2p3/2 and 2p1/2 of Ti 4+ , respectively [37]. For TiO 2 -H 2 and 3% Y-TiO 2 -H 2 , these peaks shift to low values, broaden, and become unsymmetrical compared with those of pure TiO 2 and 3% Y-TiO 2 , indicating a different bonding environment ( Figure S3) [38]. The fitting curves show that the two peaks of Ti 2p are divided into four peaks, found at 458.3 and 464.0 eV, respectively, corresponding to the 2p3/2 and 2p1/2 peaks of Ti 4+ , and at 457.8 and 463.2 eV, respectively, corresponding to the 2p3/2 and 2p1/2 peaks of Ti 3+ [39]. The Ti 3+ state indicates that TiO 2 -H 2 and 3% Y-TiO 2 -H 2 are partially reduced through H 2 reduction.   The Eg can be calculated in accordance with the formula:

Photocatalytic Test
The effect on the catalytic activities of all four samples is evaluated by the degradation of RhB under simulated sunlight irradiation, as shown in Figure 6. The UVvis absorption spectra of TiO2, 3% Y-TiO2, TiO2-H2, 3% Y-TiO2-H2 are separately displayed in Figure 6a-d, and the degradation efficiency curves are summarized in Figure 6e. For pure TiO2, RhB can be totally degraded in approximately 60 min, whereas RhB can be totally degraded by 3% Y-TiO2 in 40 min, which is shorter than that by pure TiO2. This result indicates that a proper amount of RE doping has a positive effect on photocatalytic performance. For TiO2-H2, RhB can be totally degraded only in 30 min, which is beneficial for the reduction role of TiO2. Interestingly, RhB can be totally degraded by 3% Y-TiO2-H2 in 20 min, which is less time than those of other samples. The combined action of 3% Y doping and reduction by H2 is beneficial to the photodegradation activity of TiO2, and the co-modified catalyst exhibits higher photocatalytic activity than any single-modified catalyst.
The kinetics of the photocatalytic activities of all four samples follow the first-order reaction: −ln(Ct/C0) = k1t, (2) where k1 is the pseudo-first-order reaction rate constant (min −1 ) obtained from the slope of −ln(Ct/C0) vs. t, as shown in Figure 6f,g. The k1 values of TiO2, 3% Y-TiO2, TiO2-H2, and 3% Y- TiO2-H2 are 0.0606, 0.0985, 0.1269, and 0.1746, respectively, indicating the efficient photodegradation activity of 3% Y-TiO2-H2. Therefore, the combination of Y doping and H2 reduction is responsible for the high degradation rate. The E g can be calculated in accordance with the formula: Among them, α, h, ν, and A are the absorption coefficient, Planck's constant, frequency of the incident light, and a constant, respectively. For direct and indirect transition semiconductors, n is 1/2 and 2, respectively. The value of n is 2 for the anatate TiO 2 . In Figure 5b, the E g values of TiO 2 , 3% Y-TiO 2 , TiO 2 -H 2 , and 3% Y-TiO 2 -H 2 obtained from the tangent intercept are 2.88, 2.70, 2.24, and 2.17 eV, respectively.

Photocatalytic Test
The effect on the catalytic activities of all four samples is evaluated by the degradation of RhB under simulated sunlight irradiation, as shown in Figure 6. The UV-vis absorption spectra of TiO 2 , 3% Y-TiO 2 , TiO 2 -H 2 , 3% Y-TiO 2 -H 2 are separately displayed in Figure 6a-d, and the degradation efficiency curves are summarized in Figure 6e. For pure TiO 2 , RhB can be totally degraded in approximately 60 min, whereas RhB can be totally degraded by 3% Y-TiO 2 in 40 min, which is shorter than that by pure TiO 2 . This result indicates that a proper amount of RE doping has a positive effect on photocatalytic performance. For TiO 2 -H 2 , RhB can be totally degraded only in 30 min, which is beneficial for the reduction role of TiO 2 . Interestingly, RhB can be totally degraded by 3% Y-TiO 2 -H 2 in 20 min, which is less time than those of other samples. The combined action of 3% Y doping and reduction by H 2 is beneficial to the photodegradation activity of TiO 2 , and the co-modified catalyst exhibits higher photocatalytic activity than any single-modified catalyst.

Mechanism Analysis
RE metal doping can significantly modify the electrical, physical, and chemical properties of TiO2 photocatalyst and play an effective role for improving photocatalytic performance. Compared to pure TiO2, 3% Y-doped TiO2 has a faster degradation rate of RhB, which is a ributed to the influence of Y doping. The ionic radius of Y 3+ (93 pm) is larger than that of Ti 4+ (68 pm), which is difficult to replace Ti into the TiO2 la ice directly [25]. Based on the XRD results, no diffraction peak shift is observed for 3% Y-doped TiO2, demonstrating that Y 3+ species exists at the crystal boundary or surface rather than in the inner crystalline structure of TiO2. Furthermore, the survey scanning the XPS spectra ( Figure S2) shows that a weak Y 3d peak is observed at ~158 eV, indicating the presence of Y 3+ species on the surface of 3% Y-TiO2. Based on XRD and XPS, the Y 3+ species might be deposited on the surface of TiO2. Given that the work function (Φ) of RE metals is lower than that of titanium, RE 3+ has more tendencies to a ract the e − from the sample surface The kinetics of the photocatalytic activities of all four samples follow the first-order reaction: where k 1 is the pseudo-first-order reaction rate constant (min −1 ) obtained from the slope of −ln(C t /C 0 ) vs. t, as shown in Figure 6f,g. The k 1 values of TiO 2 , 3% Y-TiO 2 , TiO 2 -H 2 , and 3% Y-TiO 2 -H 2 are 0.0606, 0.0985, 0.1269, and 0.1746, respectively, indicating the efficient photodegradation activity of 3% Y-TiO 2 -H 2 . Therefore, the combination of Y doping and H 2 reduction is responsible for the high degradation rate.

Mechanism Analysis
RE metal doping can significantly modify the electrical, physical, and chemical properties of TiO 2 photocatalyst and play an effective role for improving photocatalytic performance. Compared to pure TiO 2 , 3% Y-doped TiO 2 has a faster degradation rate of RhB, which is attributed to the influence of Y doping. The ionic radius of Y 3+ (93 pm) is larger than that of Ti 4+ (68 pm), which is difficult to replace Ti into the TiO 2 lattice directly [25]. Based on the XRD results, no diffraction peak shift is observed for 3% Y-doped TiO 2 , demonstrating that Y 3+ species exists at the crystal boundary or surface rather than in the inner crystalline structure of TiO 2 . Furthermore, the survey scanning the XPS spectra ( Figure S2) shows that a weak Y 3d peak is observed at~158 eV, indicating the presence of Y 3+ species on the surface of 3% Y-TiO 2 . Based on XRD and XPS, the Y 3+ species might be deposited on the surface of TiO 2 . Given that the work function (Φ) of RE metals is lower than that of titanium, RE 3+ has more tendencies to attract the e − from the sample surface compared with Ti ions, resulting in a reduced e − -h + pair recombination rate. In addition, 3% Y-TiO 2 with a higher specific surface area has more reactive active sites compared with pure TiO 2 , which also helps to improve the photocatalytic performance. Moreover, UV-vis DRS revealed that 3% Y-TiO 2 has higher light absorption capacities in their visible region than pure TiO 2 , resulting in lower E g values. However, an excessive amount of metal dopants may lead to a decline in photodegradation efficiency towards pollutants by reducing the yield of photoinduced e − -h + pairs ( Figure S1b). Therefore, 3% Y doping is the most suitable doping amount and selected as the subsequent research object in our work.
The reduction treatment of TiO 2 by H 2 is also one of the important methods to improve its photocatalytic performance. Compared to pure TiO 2 and 3% Y-TiO 2 , the specific surface areas of TiO 2 -H 2 significantly decreased, indicating that the reaction active sites decreased. However, TiO 2 -H 2 also had a faster degradation rate of RhB, which is attributed to the changes in microstructure caused by reduction treatment. The remarkable feature of reduced TiO 2 by H 2 is its disordered shell filled with a limited amount of O V s or Ti 3+ , which is consistent with the HRTEM and XPS results. Furthemore, the significant decrease in E g values of TiO 2 -H 2 is due to the significant increase in the sample's absorption capacity for visible light according to UV-vis DRS results. Compared with the RE metal doping, O V or Ti 3+ is a kind of self-doping of the crystal itself without introducing any impurity element, which is considered to reflect the influence of the internal modification of TiO 2 on its physicochemical and photocatalytic performance, such as tuning optical absorption, reducing E g , and increasing carrier concentration [31].
Under the simultaneous action of Y doping and reduction treatment, 3% Y-TiO 2 -H 2 exhibited a higher photocatalytic degradation ability than any single-modified catalyst. Thus, a possible photocatalytic mechanism for 3% Y-TiO 2 -H 2 nanoparticles is proposed (Figure 7). The disordered shell of reduced 3% Y-TiO 2 -H 2 is filled with a limited amount of O V s and Ti 3+ , which significantly reduces the E g of TiO 2 and remarkably increases the absorption of visible light. The formation of a defective energy level caused by O V and Ti 3+ below the conduction band of TiO 2 decreases the excitation energy, resulting in highly active photocatalysts [42]. The incorporation of Y 3+ on the surface of TiO 2 increases the charge separation by acting as an electron trapper, consequently produces more e − for reaction on the surface of the catalyst, and reduces the e − -h + recombination rate of photocatalyst. In addition to reducing carrier recombination, as a member of RE elements, Y doping broadens the range of light absorption and increases the photoactivity of TiO 2 ; this phenomenon is consistent with the DRS and photodegradation results. In conclusion, the more reactive active sites, the rapid interfacial charge transfer, and stronger optical absorption of 3% Y-TiO 2 -H 2 caused by Y doping should be the reason for the higher rate of photocatalytic reactions than those of undoped TiO 2 -H 2 [43]. Their photocatalytic process is such that, under simulated sunlight irradiation, e − is excited from VB to CB of 3% Y-TiO 2 -H 2 , and h + in VB is abandoned. Generated charge carriers react with oxygen molecules, water molecules, or OH − to produce oxidative species for the degradation of organic dye in the aqueous solution, such as hydroxyl radicals (•OH) and (•O 2 − ). Nanomaterials 2023, 13, x FOR PEER REVIEW 12 of 14 Figure 7. Schematic of the photodegradation of RhB in 3% Y-TiO2-H2 nanoparticles.

Conclusions
Y doping and the reduction treatment of TiO2 have been widely proven to be an effective way to improve their photocatalytic performance, respectively. In this work, the coupling treatment of Y doping and reduction of TiO2 can further improve the photocatalytic performance, which is be er than any individual method. In summary, pure TiO2 and 3% Y-TiO2 were prepared by a one-step hydrothermal method. Reduced TiO2-H2 and 3% Y-TiO2-H2 were obtained through the thermal conversion treatment of Ar-H2 atmosphere at 500 °C for 3 h. All samples show the single anatase phase, and no diffraction peak shift is observed. The photodegradation efficiency of pure TiO2, 3% Y-TiO2, TiO2-H2, and 3% Y-TiO2-H2 on RhB gradually increased. The 3% Y-TiO2-H2 exhibits the best photocatalytic performance for the degradation of RhB among these four samples, which can be totally degraded in 20 min. The key factors for the improved photocatalytic performance of 3% Y-TiO2-H2 could be a ributed to the synergistic effect of Y doping and the reduction of TiO2. Y metal-ion doping broadened the range of light absorption and reduced the charge recombination rates. Reduction treatment can cause TiO2 to be enveloped by disordered shells, and OVs and Ti 3+ species efficiently reduced the Eg of TiO2, remarkably increasing the absorption of light. The synergistic effect of rapid interfacial charge transfer and the stronger optical absorption of 3% Y-TiO2-H2 caused by Y doping and reduction treatment should be the reason for the high photocatalytic efficiency. The discovery of this work provides a new perspective for the improvement of other photocatalysts by combining doping and reduction to modify traditional photocatalytic materials and further improve their performance.

Conclusions
Y doping and the reduction treatment of TiO 2 have been widely proven to be an effective way to improve their photocatalytic performance, respectively. In this work, the coupling treatment of Y doping and reduction of TiO 2 can further improve the photocatalytic performance, which is better than any individual method. In summary, pure TiO 2 and 3% Y-TiO 2 were prepared by a one-step hydrothermal method. Reduced TiO 2 -H 2 and 3% Y-TiO 2 -H 2 were obtained through the thermal conversion treatment of Ar-H 2 atmosphere at 500 • C for 3 h. All samples show the single anatase phase, and no diffraction peak shift is observed. The photodegradation efficiency of pure TiO 2 , 3% Y-TiO 2 , TiO 2 -H 2 , and 3% Y-TiO 2 -H 2 on RhB gradually increased. The 3% Y-TiO 2 -H 2 exhibits the best photocatalytic performance for the degradation of RhB among these four samples, which can be totally degraded in 20 min. The key factors for the improved photocatalytic performance of 3% Y-TiO 2 -H 2 could be attributed to the synergistic effect of Y doping and the reduction of TiO 2 . Y metal-ion doping broadened the range of light absorption and reduced the charge recombination rates. Reduction treatment can cause TiO 2 to be enveloped by disordered shells, and O V s and Ti 3+ species efficiently reduced the E g of TiO 2 , remarkably increasing the absorption of light. The synergistic effect of rapid interfacial charge transfer and the stronger optical absorption of 3% Y-TiO 2 -H 2 caused by Y doping and reduction treatment should be the reason for the high photocatalytic efficiency. The discovery of this work provides a new perspective for the improvement of other photocatalysts by combining doping and reduction to modify traditional photocatalytic materials and further improve their performance.